1. Field of the Invention
The present invention relates to a coating method and an apparatus, and in particular, to a coating method and an apparatus for applying a substrate used for the production of photosensitive material and a magnetic recording medium.
2. Description of the Related Art
Each of photosensitive material and magnetic recording media is produced through a coating step of forming a coating film by applying predetermined coating liquid such as magnetic liquid on a continuously running flexible substrate (hereinafter referred to as a web). In particular, since the capacity and recording density of magnetic recording media such as magnetic recording tapes are rapidly improved as the use for broadcasting and computers, the coating technology of being able to obtain a magnetic layer having thin film thickness and smooth surface has been requested.
As methods of applying coating liquid on a web surface, there are, for example, a roll coater method, a gravure coating method, a roll coater plus doctor method, an extrusion type coating method, and a slide coater type coating method, etc.
Among extrusion type coating apparatuses, in such a coating apparatus of pressing a point of a coating head to a web as disclosed in Japanese Patent Publication No. 5-8065, it is possible to obtain a thin uniform coating layer with excluding air coming with the web by raising hydraulic pressure in the point of the coating head by using web tension. Hence, this type of apparatuses is frequently used in a field of production of magnetic recording media.
However, since there is a limit in the thickness of a possible coating layer even when coating is performed by this extrusion type of coating apparatus, there is a problem that it is not possible to sufficiently respond to a demand of obtaining a magnetic layer with thin film thickness. In addition, there is a defect that a stripe is easily generated in the coating layer due to web cut at the point of the coating head.
From this, as a coating apparatus that can obtain a thinner coating layer without a coating stripe, there is a doctor blade scraping-off type coating apparatus disclosed in Japanese Patent No. 2614119. This coating apparatus obtains a thin coating layer by scraping off an excess of coating liquid with a doctor blade arranged in a downstream side of a roll coater type applicator after applying the excessive coating liquid on a web by the applicator. In this case, though the roll coater type apparatus is used in Patent No. 2614119 as a coating apparatus to apply the excessive coating liquid on the web, it is not limited to this, but it is possible to use a supporting-member-pressurizing extrusion type apparatus, a backed-up extrusion type apparatus, a gravure slide coater type apparatus, or the like.
Nevertheless, though a conventional doctor blade scraping-off type coating apparatus is a coating apparatus that comparatively easily forms a thin coating layer without a coating stripe, there is a defect that the following coating defects may be caused when an ultrathin magnetic layer, which is unprecedented and is 2 xcexcm or less in a moist state, like those of magnetic recording media in recent years is formed.
(1) Coating thickness distribution is easily generated in a web width direction of a coating layer after scraping-off by the doctor blade due to the thickness distribution of the web itself and single-edged slack.
(2) Coating thickness distribution is easily generated in the web width direction of a coating layer after scraping-off by the doctor blade.
(3) Coating thickness variation is easily generated in a web-traveling direction of a final coating layer after scraping-off by the doctor blade.
(4) Uneven coating may be caused in the web width direction.
(5) A detailed unevenness may be generated as a coating surface gets rough.
(6) Coating stripes may happen frequently.
The above-described coating defects from items (1) to (6) were not problems particularly in the case of coating layer thickness demanded of a magnetic layer of a conventional magnetic recording medium. Nevertheless, they become obvious by making the thickness of a magnetic layer of a magnetic recording medium be 2 xcexcm or less, which is unprecedented and ultrathin, in a moist state in recent years. Hence, it is necessary to pursue occurrence causes of these coating defects and to exclude their factors.
The present invention is devised in consideration of such circumstances and aims to provide a coating method and an apparatus for being able to perform ultrathin and uniform coating on a web without causing a coating defect in scraping-off type coating.
In order to achieve the above-described object, the first form of the present invention is characterized in a coating apparatus that scrapes off an excess of coating liquid, excessively applied, with an doctor blade with relatively pressing the doctor blade on a coated side surface of a web after applying of the coating liquid, which is more excessive than desired quantity of coating liquid, on the web by a precoating apparatus while making a web travel continuously, comprising a pressure distribution adjusting device which adjusts pressure distribution in a width direction of the web pressed.
According to the first form of the present invention, a pressure distribution adjusting device adjusting the distribution of pressure, relatively pressuring the doctor blade and the web, in the width direction of the web is installed. Hence, it is possible to intentionally adjusting the scraping-off quantity of coating liquid in the width direction of the web. Owing to this, it is possible to perform the scraping-off of the coating liquid according to the thickness distribution of the web itself, the single-edged slack of the web, and the like. In particular, when a ultrathin magnetic layer, which is unprecedented and has the thickness of 2 xcexcm or less in a moist state, like a magnetic recording medium in recent years, the influence of the thickness distribution of the web itself and the single-edged slack becomes obvious, but it is possible to solve it by the present invention.
As a preferable form of the pressure distribution adjusting device, a roller is installed in at least one side of an upstream side and a downstream side of the doctor blade in a traveling direction of the web, and this roller is inclined by an inclination mechanism in a width direction of the web. Owing to this, it is possible to adjust pressure distribution in the width direction of the web. In this case, when an expander roller is used as the roller, it is possible not to linearly incline the web in the width direction of the web, but to curvilinearly incline it. Therefore, it is possible not to make the pressure distribution be linear from one edge to another edge in the width direction of the web, but to adjust the pressure distribution curvilinearly.
As another preferable form of the pressure distribution adjusting device, a blade inclination mechanism inclining the doctor blade in the width direction of the web can be also used.
Furthermore, pressure distribution in the width direction of the web can be adjusted with air by installing an air jet nozzle, which can adjust blowing pressure in the width direction of the web, on the opposite side of the doctor blade across the web.
As still another preferable form of the present invention, it is preferable to adjust the pressure distribution in the width direction of the web by a pressure distribution adjusting device so that the coating thickness distribution of a coating layer in the width direction of the web after scraping-off by the doctor blade may become 20% or less.
As a further preferable form of the present invention, it is preferable to have the configuration that coating liquid applied on the web is not exposed to the air during from the applying of the coating liquid on the web to the scraping-off of an excess of the coating liquid. Here, a sentence xe2x80x9ccoating liquid applied on the web is not exposed to the airxe2x80x9d means a state that a coating surface of the web where the coating liquid is applied is not exposed to at least the air, but it is not a problem whether an edge in the width direction of the web is exposed to the air.
In order to achieve the above-described object, the fifth form of the present invention is characterized in a coating method for scraping off an excess of coating liquid, excessively applied, with a scraping-off member with relatively pressing the scraping-off member on a coated side surface of a web after applying of the coating liquid, which is more excessive than desired quantity of coating liquid, on the web by a precoating apparatus while making a web travel continuously, comprising a step of making coating thickness distribution in the width direction of the web before an excess of coating liquid is scraped off by the scraping-off member be 20% or less.
Like the fifth form of the present invention, in the case of a post-measurement system of coating method for forming a final coating layer by scraping off an excess by a scraping-off member after excessive applying is performed by a precoating apparatus beforehand, it has been considered that the final layer thickness of the coating layer and the thickness distribution in the width direction of the web are determined by the quality of scraping-off with the scraping-off member. Hence, heretofore, the scraping-off technology has been researched.
However, when researching factors that the final thickness distribution of the coating layer was generated, the present inventor found that not only the quality of scraping-off with the scraping-off member greatly influences, but also the coating thickness distribution of the web in the width direction before the scraping-off greatly influences the thickness distribution of a final coating layer in the width direction of the web after the scraping-off. In particular, when an ultrathin magnetic layer, which is unprecedented and has the thickness of 2 xcexcm or less in a moist state, like a magnetic recording medium in recent years, the influence of the coating thickness distribution of the web in the width direction before scraping-off becomes obvious.
The present invention is devised on the basis of this finding, and makes the coating thickness distribution in the width direction of the web before scraping-off of the excess of coating liquid, excessively applied in a precoating apparatus, by scraping-off member 20% or less. Owing to this, with maintaining features of the scraping-off type coating method suitable for obtaining an ultrathin coating layer, it is possible to remarkably reduce the thickness distribution of the final coating layer in the width direction of the web. Hence, this is particularly effective as a coating method of obtaining an ultrathin magnetic layer of 2 xcexcm or less in a moist state. In addition, as the scraping-off member used here, a doctor blade or a rod (a flat rod, a wire rod, or a grooved rod) can be suitably used.
Moreover, as a method of making the coating thickness distribution in the width direction of the web before scraping-off be 20% or less, any one of a method of providing a coating thickness distribution adjusting function for a precoating apparatus itself, or a method of providing the coating thickness distribution adjusting function separately from the precoating apparatus is acceptable.
As a preferable form of adjusting the coating thickness distribution in the width direction of the web before scraping-off, it is possible to suitably use a method of adjusting its slit width by using a coating head, having a slit, such as a supporting-member-pressurizing extrusion type coating head or a backed-up extrusion type coating head as the precoating apparatus. In this case, in the supporting-member-pressurizing extrusion type coating head, it is possible to adjust the coating thickness distribution by inclining support rollers installed in the upstream and downstream sides of the coating head. Moreover, it is also good to use the inclination of the support rollers and slit width adjustment together. In short, the method is good so long as the method can make the coating thickness distribution in the width direction of the web before scraping-off by the scraping-off member be 20% or less.
In addition, also in the case of the fifth form, it is preferable to have the configuration that coating liquid applied on the web is not exposed to the air during from the applying of the coating liquid on the web to the scraping-off of an excess of the coating liquid.
In order to achieve the above-described object, the seventh form of the present invention is characterized in a coating method for scraping off an excess of coating liquid, excessively applied, with a scraping-off member with relatively pressing the scraping-off member on a coated side surface of a web after applying of the coating liquid, which is more excessive than desired quantity of coating liquid, on the web by a precoating apparatus while making a web travel continuously, comprising a step of making variance of coating thickness in the traveling direction of the web in a coating layer after scraping-off of an excess of coating liquid by the scraping-off member be 20% or less.
In the case of a post-measurement system of coating method for forming a final coating layer by scraping off an excess by a scraping-off member after excessive applying is performed by a precoating apparatus beforehand, like the seventh form of the present invention, it has been considered that the final layer thickness of the coating layer and the thickness variance in a traveling direction of the web are determined by the quality of scraping-off with the scraping-off member. Hence, heretofore, the scraping-off technology has been researched.
However, when researching factors that the thickness variance of the final coating layer in the traveling direction of the web was generated, the present inventor found that not only the quality of scraping-off with the scraping-off member greatly influences, but also respective factors of thickness variance in the traveling direction of the web before an excess of coating liquid is scraped off by the scraping-off member, variance of traveling speed of the web, tension variance of the web in a portion of the web (a scraping-off portion) where pressure is given by the scraping-off member, and viscosity of the coating liquid greatly influence the thickness variance of the final coating layer in the traveling direction of the web. In particular, when an ultrathin magnetic layer that is unprecedented and has the layer thickness of 2 xcexcm or less in a moist state, the influence of the above-mentioned respective factors becomes obvious. Then, it was found that it is important to make coating thickness variance in the traveling direction of the web in the final coating layer after an excess of coating liquid is scraped off by the scraping-off member be 20% or less, so as to maintain and improve the performance of the coating layer, and in particular, the performance of an ultrathin magnetic layer of 2 xcexcm or less in a moist state.
The present invention is devised on the basis of this finding, and makes the coating thickness variance in the traveling direction of the web after scraping-off of the excess of coating liquid, excessively applied in a precoating apparatus, by the scraping-off member 20% or less. Owing to this, with maintaining features of the scraping-off type coating method suitable for obtaining an ultrathin coating layer, it is possible to remarkably reduce the thickness variance of the final coating layer in the width direction of the web. Hence, it is possible to maintain the quality of the ultrathin magnetic layer, which is unprecedented and is 2 xcexcm or less in the moist state, like those of magnetic recording media in recent years. As the scraping-off member used here, a doctor blade or a rod (a flat rod, or a grooved rod) can be suitably used.
Moreover, as a preferable form of a method of making the coating thickness variation of the coating layer in the traveling direction of the web after scraping-off by the scraping-off member be 20% or less, there are a method of making coating thickness variance in the traveling direction of the web before scraping-off an excess of coating liquid by the scraping-off member be 20% or less, a method of making the variance of traveling speed of the web be 20% or less, a method of making the variance of tension in a traveling direction of the web in a web portion, where pressure is given by the scraping-off member, be 10% or less, and a method of making the variance of viscosity of coating liquid be 20% or less, but it is preferable to combine these methods properly.
In addition, also in the case of the seventh form, it is preferable to have the configuration that coating liquid applied on the web is not exposed to the air during from the applying of the coating liquid on the web to the scraping-off of an excess of the coating liquid.
In order to achieve the above-mentioned object, the twelfth form of the present invention is characterized in a coating method for scraping off an excess of coating liquid with relatively pressing a scraping-off member on a coated side surface of a web after applying of the coating liquid, which is more excessive than desired quantity of coating liquid, on the web by a precoating apparatus while making a web travel continuously, comprising a step of preventing a crimp of the web in a position of the scraping-off member.
The inventor of the present invention paid attention to the crimp of the web in a position of the scraping-off member as a cause of the uneven thickness generated in the width direction of the web. The crimp of the web used here means a wrinkle in a longitudinal direction that is generated by the web being elastically transformed, as shown in FIG. 31. Heretofore, even if some crimp was generated in a web, there was no large influence on a coated face since the thickness of a coating layer was thick. However, the present inventor found that even some crimp influences directly on the coating surface since the influence of the crimp grows in a position of the scraping-off member when forming an ultrathin coating layer, in particular, a coating layer with the thickness of 2 xcexcm or less in a moist state. The present invention is devised on the basis of this finding, and performs ultrathin and even coating by preventing the crimp of the web in a position of the scraping-off member.
As a method of preventing the crimp, there is a method of adjacently arranging crimp prevention members such as expander rollers, grooved rollers, and edge guiders, before and after the scraping-off member. In this case, it is possible to effectively prevent the crimp of the web in the position of the scraping-off member by arranging the crimp prevention members within a range of not exceeding 1000 mm before and after the scraping-off member in the traveling direction of the web.
Moreover, as another crimp preventing method, there is a method of adjacently arranging a member, having a crimp prevention effect such as support rollers, backup rollers, and coating heads before and after the scraping-off member. In this case, it is possible to effectively prevent the crimp of the web in the position of the scraping-off member by arranging the members, each having a crimp prevention effect, within a range of not exceeding 200 mm before and after the scraping-off member in the traveling direction of the web.
In addition, also in the case of the twelfth form, it is preferable to have the configuration that coating liquid applied on the web is not exposed to the air during from the applying of the coating liquid on the web to the scraping-off of an excess of the coating liquid.
In order to achieve the above-described object, the sixteenth form of the present invention is characterized in a coating apparatus that scrapes off an excess of coating liquid with relatively pressing a doctor blade on a coated side surface of a web after applying of the coating liquid, which is more excessive than desired quantity of coating liquid, on the web by a precoating apparatus while making the a web travel continuously, wherein thickness of the doctor blade in a traveling direction of the web is 1 mm to 30 mm.
The inventor of the present invention paid attention to the cohesion of coating liquid as a main cause of face roughness being generated when ultrathin coating in 2 xcexcm or less was performed in a moist state. Then, from the fact that a coating surface got rougher as coating liquid had stronger cohesion, the inventor found that unevenness appears on a coating surface by the cohesion of the coating liquid and the face roughness is generated. Moreover, the inventor found that what is necessary so as to control this face roughness is to untie the aggregation in the coating liquid by giving an appropriate shearing force with a doctor blade. The present invention is devised on the basis of such a finding, and provide a coating apparatus that can obtain an excellent coating surface by untying the cohesion of coating liquid, which influences an ultrathin coating layer, by giving an appropriate shearing force with a doctor blade.
As a method of giving an appropriate shearing force with a doctor blade, a method of making the thickness of the doctor blade (hereinafter, it is called a xe2x80x9cblade thicknessxe2x80x9d) in the traveling direction of a web within an appropriate range, and a method of making a shape of an apical surface (a surface facing the web) of the doctor blade an appropriate shape are conceivable. For example, when ultrathin coating in 2 xcexcm or less is performed in a moist state, the blade thickness of the doctor blade can be made within a range of 1 mm to 30 mm. Since the aggregation in the coating liquid cannot be untied enough if the blade thicknesses is smaller than this range, the face roughness is generated. On the contrary, since the coating liquid scraped off by the doctor blade does not flow smoothly if the blade thickness is larger than this range, uneven coating is generated as a result. Hence, it is possible to obtain an ultrathin coating surface without uneven coating and surface roughness by making the blade thickness of the doctor blade be 1 mm to 30 mm.
Moreover, as a shape of the apical surface of the doctor blade that can effectively give the shearing force, a shape of combining a plurality of planes, a shape of combining a plurality of curved surfaces, or a shape of combining a plane and a curved surface is conceivable. For example, if the radius of curvature is made to become gradually small from an upstream side in the traveling direction of the web to a downstream side when the shape of combining a plurality of curved surfaces is adopted, it is possible to give the shearing force effectively, and hence, it is possible to obtain an ultrathin coating surface without face roughness and uneven coating.
In addition, also in the case of the sixteenth form, it is preferable to have the configuration that coating liquid applied on the web is not exposed to the air during from the applying of the coating liquid on the web to the scraping-off of an excess of the coating liquid.
In order to achieve the above-described object, the seventeenth form of the present invention is characterized in a coating method for scraping off an excess of coating liquid with relatively pressing a scraping-off member on a coated side surface of a web after applying of the coating liquid, which is more excessive than desired quantity of coating liquid, on the web by a precoating apparatus while making a web travel continuously, the evaporation of a solvent vaporizing from the coating liquid on the web is controlled between from the precoating apparatuses to the scraping-off member lest a concentration of a solid component in the coating liquid applied on the web should exceed 80% at a position of the scraping-off member.
The inventor of the present invention found that a main cause that coating stripes happen frequently when an excess of coating liquid is scraped off in the ultrathin coating with the thickness of 2 xcexcm or less in a moist state is the concentration of a solid component in the coating liquid on the web in a position of the scraping-off member. That is, the inventor found that the main cause of the coating stripe is that a solvent evaporates from the coating liquid applied on the web by the precoating apparatus and the concentration of the solid components rises, and in particular, when the concentration of the solid component in the position of the scraping-off member exceeds 80%, the coating stripe happens frequently in the ultrathin coating in the thickness of 2 xcexcm or less. The present invention is devised on the basis of such a finding, and performs ultrathin and even coating by performing control so that the concentration of the solid component in the coating liquid may not exceed 80% in the position of the scraping-off member. The concentration of the solid components in the coating liquid can be adjusted by controlling the evaporate of a solvent (hereinafter, it is called a xe2x80x9csolvent evaporatexe2x80x9d) from coating liquid on the web between from the precoating apparatus to the scraping-off member. In addition, the solvent evaporate may be set to be, for example, 50% or less of the quantity of the solvent (hereinafter, it is called an xe2x80x9cinitial contentxe2x80x9d) contained in the coating liquid that is applied by the precoating apparatus.
As a preferable control method of the solvent evaporate, it is good to set the time, necessary for scraping off the coating liquid, applied by the precoating apparatus, by the scraping-off member (i.e., xe2x80x9cevaporation timexe2x80x9d of the solvent) to be, for example, 0.2 seconds or less. Owing to this, since it is possible to reduce the solvent evaporate, it is possible to make the concentration of solid components in coating liquid be 80% or less.
Moreover, as another control method of the solvent evaporate, it is good to lower the temperature of coating liquid applied by a precoating apparatus by 30xc2x0 or more than a boiling point of a solvent with the lowest boiling point (hereinafter, this is called a xe2x80x9clowest boiling point solventxe2x80x9d) among contained solvents in the coating liquid. Owing to this, since the coating liquid in low temperature is applied on the web, it becomes difficult for solvents in the coating liquid to evaporate, and hence, it is possible to control the concentration of the solid components in the coating liquid at 80% or less.
Moreover, as still another control method of the solvent evaporate, it is good to lower the temperature of the web in the position of the precoating apparatus by 30xc2x0 or more than a boiling point of the lowest boiling point solvent. Owing to this, since the coating liquid is applied on the web in low temperature, it becomes difficult for solvents in the coating liquid to evaporate, and hence, it is possible to control the concentration of the solid components in the coating liquid at 80% or less.
In addition, also in the case of the eighteenth form, it is preferable to have the configuration that coating liquid applied on the web is not exposed to the air during from the applying of the coating liquid on the web to the scraping-off of an excess of the coating liquid.